1. Field of the Invention
The present invention relates to an optical information recording medium, and more specifically the present invention relates to a heat mode recordable optical information recording medium.
2. Description of the Related Art
An optical information recording medium (optical disc) that allows recording of information only one time by use of a laser beam is known. The optical disc is also called a recordable CD (so-called CD-R), and typically has a transparent disc-like substrate, a recording layer comprising an organic dye, a reflection layer comprising metal such as gold, and a resinous protection layer (cover layer) in this order. The information is recorded on the CD-R as follows. That is, a laser beam in a near-infrared region (usually a laser beam having a wavelength in the neighborhood of 780 nm) is irradiated to the CD-R, an irradiated portion of the recording layer absorbs the light and is locally heated, thereby causing a physical or chemical change thereof (for instance, formation of a pit), and optical characteristics of the portion are changed. As a result, the recording of the information is realized. On the other hand, the information is also read (reproduced) by irradiating a laser beam having the same wavelength as that of the laser beam used in recording on the CD-R and by detecting difference in reflectance of a portion where the optical characteristics of the recording layer have been changed (recorded portion) and a portion where the optical characteristics have not been changed (unrecorded portion).
In recent years, an optical information recording medium higher in recording density is in demand. In response to such demands, an optical disc called a recordable digital versatile disc (so-called DVD-R) has been proposed (e.g. Nikkei New Media, extra issue for “DVD” in 1995). The DVD-R has the following structure. Normally, a recording layer containing an organic dye, a reflection layer, and a protection layer are successively disposed in this order on a transparent disc-like substrate on which a guide groove (pre-groove) for tracking an irradiated laser beam is formed having a width (0.74 to 0.8 μm) as narrow as one half or less that of the CD-R. Two thus formed discs are adhered to each other with the recording layers positioned inward, or the disc and a disc-like protection substrate having the same shape as the disc are adhered with the recording layer positioned inward. The information is recorded on and reproduced from the DVD-R by irradiating a visible laser beam (usually a laser beam having a wavelength in the range of 630 to 680 nm) and can be recorded with a density higher than that of the CD-R.
Recently, networks, such as the Internet, or the like and high-vision TVs are rapidly spreading. Furthermore, High Definition Television (HDTV) broadcasting is to start in the near future. Under such circumstances, a large capacity recording medium that can cheaply and simply record image information is in demand. Although the DVD-R sufficiently plays a role as the large capacity medium under the present circumstances, since the demand for larger capacity and higher density is increasing more and more, it is necessary to develop a recording medium that can cope with these demands. Accordingly, an even larger capacity recording medium that can densely record with light having a wavelength even shorter than that of the DVD-R is under development.
For instance, Japanese Patent Application Laid-Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028 disclose a recording or reproducing method wherein, in an optical information recording medium having a recording layer containing an organic dye, a laser beam having a wavelength of 530 nm or less is irradiated from a recording layer side to a reflection layer side, and information is thereby recorded or reproduced. In the above methods, a laser beam having a blue color (wavelength: 430 nm, 488 nm) or a blue green color (wavelength: 515 nm) is irradiated to an optical disc having a recording layer that contains one of porphyrin compounds, azo dyes, metal azo dyes, quinophthalone dyes, trimethinecyanin dyes, dicyanovinylphenyl skeletal dyes, cumarin compounds, and naphthalocyanin compounds, and the information is thereby recorded or reproduced.
On the other hand, phase-change optical discs are known among DVDs. In the phase-change optical disc, a layer of an alloy, such as GeSbTe or the like, is adopted as a recording layer, the recording layer is instantaneously heated with a laser beam so as to cause a phase change from a crystalline state to an amorphous state and thereby cause a change in reflectance thereof, and the change in reflectance is used to record and reproduce information. Recently, a DVR system in which a blue-violet laser beam is used with the phase change DVD to record and reproduce has been published ([ISOM2000]: pp. 210 to 211). With the advent of this system, a certain degree of success in attaining a higher density can be achieved.
Recently, a DVR in which an organic substance is used in a recording layer thereof has been proposed. However, in the DVR in which an organic substance is used in the recording layer, since the configuration thereof is different from that of the DVD-R and CD-R, even when, for instance, a dye used in an HD-DVD, which does not have a cover layer, is used, a carrier to noise ratio (C/N) is small, thereby making recording impossible.